This application claims the priority of European patent document EP 09 005 512.0, filed Apr. 20, 2009, the disclosure of which is expressly incorporated by reference herein.
The present invention is directed to a method of amending navigation data of a global navigation system that comprises a plurality of space vehicles which transmit information to a device for position detection, with each space vehicle having at least one clock. In particular, the method according to the invention reduces the impact of jumps in the space vehicle clock frequency on the position detection device.
Space vehicle based navigation systems, such as satellite navigation systems, generally rely on very stable satellite clock performance to allow accurate satellite clock behavior prediction, which is required to model accurately the satellite clocks at the user level. The user predicts the clock behavior via related transmitted clock parameters, which are estimated on ground, based on measurements over long intervals (e.g., one to two days). International Patent Document WO 2006/032422 A1 discloses a method and apparatus for providing integrity information for users of a global navigation system. The disclosure of this document is fully incorporated by reference herein.
Unpredictable events, which cannot be modeled, and thus cannot be compensated or predicted at user level, directly degrade achievable ranging accuracy, since such events would cause additional range errors to occur. Early test results, including factory tests of the European Galileo satellites, showed that Rubidium clocks, which are used in the Galileo test satellites (GIOVE-A and GIOVE-B) and which will be used during the In Orbit Validation (IOV) and in the Full Operational Constellation (FOC) of Galileo, are subject to unpredictable frequency jumps, typically one to two events per month. Such jumps affect the ranging accuracy by approximately 1 m to 10 m, and thus have a major impact on all Galileo services.
For typical positioning services like the Open Service (OS) this effect is less critical, since not all users are always affected and jumps also only occur from time to time. Therefore the effect can be compensated or at least mitigated by averaging over Galileo's system lifetime (i.e., 20 years); however, it will degrade the Open Service performance.
For integrity users like Safety-of-Life (SOL) and Public Regulated Service (PRS) users, such averaged compensation is not possible, since a certain accuracy of the individual ranging signal must be ensured with very high confidence. Thus, all integrity information for each satellite and for all of the time, would need to be a-priori degraded in order to take into account the non-predictable events, which would of course jeopardize the related major Galileo services in terms of their availability.
If the unpredictable events like satellite clock frequency jumps were detected on ground, and if warnings could be broadcast to all users accordingly, the integrity services availability degradation could be significantly compensated or reduced, respectively. Unfortunately, however, since such events typically affect the ranging signals below the ground integrity detection barrier thresholds (around 5 m vs. typical range errors around 2 m), most satellite clock frequency jumps cannot be detected on the ground, and therefore the integrity information would need to be a-priori increased accordingly, with significant integrity service availability degradation.
Therefore, it is an object of the present invention to provide a method of amending navigation data in a global navigation system that includes a plurality of space vehicles that transmit information to a device for position detection, each space vehicle comprising at least one clock, wherein the impact of space vehicle clock frequency jumps on the device for position detection is reduced significantly.
This and other objects are achieved by the method according to the invention, in which the impact of space vehicle clock frequency jumps on the device for position detection is reduced by the steps of:                1a) receiving navigation signals from space vehicles of a first group of space vehicles that have clocks in which no frequency jumps occur;        1b) checking                    1b1) whether navigation signals received from a sufficient number of space vehicles of said first group of space vehicles are available for calculating a navigation solution; and            1b2) whether the integrity risk calculated with the navigation signals received from the space vehicles of said first group of space vehicles is less than or equal to a predetermined acceptable maximum integrity risk;                        1c) continuing with calculating a navigation solution or with a critical operation if the conditions of steps 1b1) and 1b2) are fulfilled, or otherwise, continuing with step 1d);        1 d) receiving navigation signals from space vehicles of a second group of space vehicles having clocks in which frequency jumps can occur;        1e) adding said navigation signals received from a space vehicle of said second group of space vehicles to said navigation signals received from the space vehicles of said first group of space vehicles, with integrity and in a safe manner;        1f) checking whether the integrity risk calculated for all combinations of the navigation signals received from the space vehicles of said first group, together with the sub-set of said second group of space vehicles with data integrity, is less than or equal to a predetermined acceptable maximum integrity risk;        1g) continuing with calculating the navigation solution or a critical operation if the condition of step 1f) is fulfilled; or otherwise, adding navigation signals received from another space vehicle of said second group to the navigation signals used in step 1f), with integrity and in a safe manner, and continuing again with step 1f).        
Consequently, the respective integrity risks calculated for all combinations of the navigation signals received from the space vehicles of said first and second groups of space vehicles must be lower than the predetermined allocated integrity risk, because it is unknown whether one of the signals received from said second group of space vehicles (and if so, which one) was just affected by a frequency jump or will be affected by it in the near future. Only such a procedure considering all combinations will deliver a result that has integrity (i.e., it is reliable).
The core idea of the first inventive solution is thus to consider primarily signals from satellite clock sources that do not jump. The effect of satellite clock frequency jumps and other similar events (if they cannot be avoided at satellite level, or detected at ground segment level with removal at user level through transmitted alerts) is thus reduced by avoiding the use of affected satellites at user level. This can be realized through suitable user systemic modifications.
The invention thus limits the impact on the projects Galileo In Orbit Validation (IOV) and the Full Operational Constellation (FOC) to a minimum, since neither space segment design changes nor ground segment modifications are required (which typically significantly impact cost and schedule). Only additional analyses and concept modifications at system level are required, together with the relevant test user updates, which do not affect the related mentioned projects significantly.
The basic idea of the invention is thus to overcome, at user algorithm level, the problem that small errors (on the order of a few meters) which are caused by satellite clock frequency jumps for example, can neither be avoided at satellite level, nor be detected by the Galileo ground integrity monitoring concept. This is done by related user integrity process modifications that endeavor to avoid to a maximum extent the usage of potentially affected signals, or to consider only such signals as would have acceptable impact at user level from integrity service availability point of view.
Such modified user algorithms do not require significant system, space or ground segment design changes, since only the final user algorithm implementation is affected. Furthermore, minor data dissemination adjustments (i.e., updates of the signal-in-space interface control document [SIS-ICD]) could also be considered to further improve the process modification compensation. Thus, the invention requires almost no modifications for the IOV/CDE1 and FOC projects in order to compensate for the most critical frequency jump behavior.
Preferably, the navigation signals received from a space vehicle of said second group are added to the navigation signals, with integrity and in a safe manner, by putting them to the ground segment detection threshold in step 1e). The ground detection threshold represents the smallest error, (i.e., jump) for said second group of space vehicles, that the ground integrity monitoring function is able to detect (and to send a warning to the user immediately). “Putting the navigation signal to the threshold” means to consider the signal and the related integrity information as having been fully affected by a jump or other error source up to the detection threshold; this technique ensures the signal is considered in a manner which preserves its integrity, since it is assumed that a jump occurred with a maximum possible error that is just smaller than can be detected by the ground segment.
Alternatively, the navigation signals received from a space vehicle of the second group can be added to the navigation signals, in a safe manner which preserves the signal integrity, by inflating the integrity information of the signal in space accuracy (SISA) in step 1e) to ensure overbounding, with integrity, of the real signal in space error of said signal by the used inflated SISA information. Inflating the integrity information of the signal means that SISA is inflated in such a way that such integrity information still properly (i.e., with integrity) overbounds the real error, even if the signal has just jumped. The inflation must be done in such a way that even the worst possible jump magnitude (i.e., maximum error) is covered. The latter alternative approach could be considered if the SISA inflation provides better integrity service availability compared to the above described conservative detection threshold approach, and vice-versa.
In a further embodiment of the method according to the invention, the SISA is inflated as a function of navigation data age in order to reduce the required integrity information inflation of said signal. That is, the effect of the jump and the related imposed error increases with the age of the latest received satellite clock parameters that are used to model the satellite clock behavior. Right after a jump the “old” parameter still fit the new clock behavior (after the jump); only after some time does the real clock drift away from the estimated (modeled) clock behavior, and the imposed error increases accordingly. If only signals with “young” navigation data (which carry also the clock parameters) are considered, the SISA does not need to be inflated to cover the worst possible maximum error, but only to cover the maximum error that could occur according to the navigation data age.
Further preferably, the space vehicles of said first group of space vehicles are provided with clocks working according to the principle of passive hydrogen maser (PHM). These PHM clocks are known as not having frequency jumps.
According to a second aspect of the invention, the impact of space vehicle clock frequency jumps on the position detection device is reduced by the steps of:                a) receiving navigation signals from all available space vehicles;        b) determining the integrity risk of the navigation signals received from the space vehicles in step a);        c) sorting the received navigation signals for the smallest individual integrity risk in a sorting list;        d) determining a first combination of navigation signals from a predetermined number of space vehicles with the smallest individual integrity risks;        e) checking whether the overall integrity risk calculated with the navigation signals received from the space vehicles of the first combination of space vehicles is equal to or lower than a predetermined acceptable maximum risk;        f) for a sufficient number of available signals, or for the first iteration cycle, considering with integrity the navigation signals from the combination of space vehicles, and calculating the navigation solution or the critical operation, respectively if the condition of step e) for all possible safe combinations is fulfilled; or otherwise, adding to the subset used in step e), in a manner that is safe and preserves signal integrity, navigation signals received from the next space vehicle of the sorting list, and continuing again with step f).        
The core idea of this second aspect of the invention is to consider only combinations of measurements that would allow for service usage, even in safe consideration of jumping signals.
Preferably, the navigation signals received from each of the space vehicles of the combination are added to the navigation signals, with integrity and in a safe manner, by putting them to the ground segment detection threshold in step f).
Alternatively the navigation signals received from each of the space vehicles of the combination may be added to the navigation signals, with integrity and in a safe manner, by inflating the integrity information of the signal in space accuracy (SISA) in step f) to ensure the integrity of overbounding of the real signal in space error of said signal by the used inflated SISA information.
Further preferably, the SISA is inflated as a function of navigation data age in order to reduce the required integrity information inflation of said signal.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.